1. Technical Field
The present invention relates to a liquid jet apparatus and printing apparatus arranged to print predetermined letters and images by emitting microscopic droplets of liquids from a plurality of nozzles to form the microscopic particles (dots) thereof on a printing medium.
2. Related Art
An inkjet printer as one of such printing apparatuses Inkjet printers are generally low-price and provide high quality color prints Inkjet printers are not only used in offices but also used by general users as well in conjunction with the widespread use of personal computers and digital cameras.
In recent inkjet printers, printing in fine tone is required. Tone denotes a state of density of each color included in a pixel expressed by a liquid dot. The size of the dot corresponds to the color density of each pixel and is called a tone grade. The number of tone grades capable of being expressed by a liquid dot is called a tone number. Fine tone denotes that the tone number is large. In order to change the tone grade, it is required to modify a drive pulse to an actuator for a liquid jet head. When a piezoelectric element is used as the actuator, since the amount of displacement of the piezoelectric element (distortion of a diaphragm, to be precise) becomes large when the voltage value applied to the piezoelectric element becomes large, the tone grade of the liquid dot can be changed very accurately.
Therefore, in JP-A-10-81013 a plurality of drive pulses with different wave heights are combined and joined. The drive pulses are commonly output to the piezoelectric elements of the nozzles of the same color provided to the liquid jet head. A drive pulse corresponding to the tone grade of the liquid dot to be formed is selected for every nozzle out of the plurality of drive pulses. The selected drive pulses are supplied to the piezoelectric elements of the corresponding nozzles to emit droplets of the liquid different in amount, thereby achieving the required tone grade of the liquid dot.
The method of generating the drive signals (or the drive pulses) is described in FIG. 2 of JP-A-2004-306434. Specifically, the data is retrieved from a memory storing the data of the drive signal. The data is converted into analog data by a D/A converter, and the drive signal is supplied to the liquid jet head through a voltage amplifier and a current amplifier. The circuit configuration of the current amplifier is, as shown in FIG. 3 of JP-A-2004-306434, composed of push-pull connected transistors, and the drive signal is amplified by a so called linear drive. However, in the current amplifier with such a configuration, the linear drive of the transistor is inefficient, a large-sized transistor is required as a measure against heating of the transistor, and moreover, a heat radiation plate for cooling the transistor is required. Thus, a disadvantage of growth in the circuit size arises, and among others, the size of the heat radiation plate constitutes a great barrier to layout design.
In order to overcome this disadvantage in the inkjet printer described in JP-A-2005-35062, the drive signals are generated by controlling a reference voltage of a DC/DC converter. In this case, since a DC/DC converter with good efficiency is used, the heat radiation unit for cooling can be eliminated. Moreover, since a pulse width modulation (PWM) signal is used, a D/A converter can be configured with a simple low-pass filter. As a result, the circuit size can be reduced.
However, obtaining a preferable waveform of the drive signal for ejecting an ink droplet from the inkjet head creates a problem. More specifically, obtaining a rapid rising or falling waveform is difficult since the DC/DC converter is, by nature, designed to generate a constant voltage in a head drive device of the inkjet printer described in JP-A-2005-35062 using the DC/DC converter described above. Further, there is also a problem that the heat radiation plate is too large to substantially complete the layout, particularly in a line head printer having a large number of nozzles in a head drive device of the inkjet printer described in JP-A-2004-306434 for amplifying the current of an actuator drive signal with a push-pull transistor.